A catalog of non-toxic building systems.
Monday, April 25, 2005Adobe - The Most Common Architecture On Earth
The most well known of the twelve basic earthen construction methods, adobe is the evolutionary descendent of cob construction and has a history of about 10,000 years with equivalents in both the old and new world. Adobe block construction is believed to have been one of human civilization's first mass production industries. It is said that half the population of the planet lives in homes made of earth and most of those are based on adobe construction. In the US adobe is found mostly in the drier climate southwestern states where the adobe building traditions of native Americans merged with those of the Spanish settlers and evolved into the 'mission style' and 'pueblo style' commonly seen today and mimicked with other non-sustainable materials throughout the region. In Europe its use has been confined to the warm dry southern regions where the technology migrated from the Middle East through Moorish/Persian/Turkish influence, being most common in Spain. Elsewhere, it is common throughout most of Asia, Africa, and the Middle East and is in routine use to the present day.
Adobe construction is based on the use of prefabricated clay-rich mud and plant fiber blocks formed with wood or metal forms or sometimes by machine and which are air/sun-dried or sometimes artificially baked. These are stacked into thick walls atop a rammed earth, stacked stone, or concrete foundation and bonded with a moist adobe mortar then finished with an adobe plaster (plain, dyed, or waxed), sometimes a white lyme plaster, or sometimes tile. Adobe block was a great improvement in technology over cob because it allowed for more flexibility in sourcing of materials and schedule of construction, allowed mass production of modular building components, and reduced the amount of water needed at the construction site.
Roofing is traditionally flat and based on the use of rustic wood beams or peeled logs set into the adobe walls. The beams support a ceiling of wood plank or 'latilla'; short thin peeled logs arrayed in a diagonal pattern. Sometimes a thick plaster finish is applied between the beams to create a smooth ceiling look. This use of single-piece beams tends to limit room spans to about a 20' maximum, but as with cob, post & beam structures are often employed to support larger spans and multiple storeys. Roofs are finished with plank or plywood sub-roof topped by tar and a gravel ballast. This is highly failure prone, though, and more modern adobe homes commonly employ built-up roofing akin to commercial buildings or elastomeric membranes and coatings often with a foam insulation underlayment. Metal roofing works well and reduces maintenance costs by virtue of the protection affored by the roof overhang but usually needs a very different kind of ceiling system. Adobe block can also be used to form arches and domes to preclude the use of beams and compound roofing. But this involves highly skilled labor and is limited to very dry environments unless elastomeric coatings or hydrostatic plasters are employed to protect these earthen roofs from wear.
Flooring is commonly poured adobe, stone, or tile but most any other flooring materials can be employed. Since a slab foundation is commonly used in modern 'pueblo style' homes, this is a common underfloor material. Radiant floor heating, its tubing cast in place with a slab, is the norm in higher value contemporary adobe homes as is the use of passive solar heating.
Most adobe homes rely on rectilinear forms as reflective of the block geometry but radial forms are not uncommon. Traditional peublo style homes follow a pattern of conjoined single-room boxes with small windows and doors. Mission style derives from the Ancient Roman villa and is often organized around a central courtyard or series of walled enclosures, an approach also employed in the Chinese tradition. Building structure often merges with exterior walls to create outdoor enclosures and courts. More contemporary designs -particularly those employing passive solar heating- employ more elaborate or, conversely, simpler arrangements with large open-plan spaces fronted by large windows.
Like cob, formed-in features such as shelving, alcoves, and seating/sleeping platforms are commonly used with adobe but the regular shape of the adobe block does not lend itself to as much sculptural freedom as cob. However, cob readily integrates with adobe allowing for more elaborate details if desired, albeit retrofit to the block structure.
As a non-toxic housing approach, adobe is excellent for the same reasons that all earthen building methods are. The materials are natural, breathable, hydroscopic, and comfortable thanks to their high thermal mass properties. The common use of radiant floor heating is a definite plus for the non-toxic home. This is the preferred heating technology for non-toxic housing and integrates well with active solar heating systems. But there are caveats. Modern factory-produced adobe blocks typically feature stabilizer additives in the form of small quantities of portland cement or asphalt. The latter is to be strictly avoided for the non-toxic home but is very difficult to identify in the completed adobe structure. Traditional native American adobe block sometimes uses a natural prickley-pear cactus juice as a stabilizer.
Modern adobe construction also often features a dual-wall system where an air gap is made within the wall to add insulation and host utilities conduits. Some forms of insulation used to fill this gap may not be suited to non-toxic housing and, again, determining the type used can be difficult with the completed building. Also, some of these can be hydrostatic which ruins the hydroscopic qualities of the earthen material and can lead to humidity build-up and mold growth. The same problem results from combining inappropriate wall finishing materials -such as polymer based stuccos- which can trap moisture.
Like almost all earthen construction methods, labor overhead for adobe block construction is very high and unless one can build a home oneself realizing any savings over other construction methods is unlikely. Maintenance is also high, though not as great as cob due to the stabilizers common in modern adobe.
For more info see; The Adobe Alliance
Cob - The Original Earthen Building Method
With a history going back in excess of 10,000 years and examples common in both the new and old world, cob construction is probably the oldest of the twelve basic methods of earthen construction and possibly the oldest method of construction in human civilization. However, most of the methodology employed in its use today originates in the British traditions of cob construction some hundreds to thousands of years old. Cob housing has become very popular among sustainable architecture enthusiasts as a result of the work of the Cob Cottage Company in Oregon which reintroduced the technology to the US and established its contemporary style. Their web site and books offer the definitive source of information on the technology.
Cob homes are often attributed with a serene ambiance or neo-primitive charm as a consequence of their combination of natural materials and organic/rustic design. Very popular with builders of an artistic and/or counter-cultural bent, designs often diverge into wild free-form variations featuring elaborate hand-sculptured details.
Cob construction is a primitive form of adobe construction based on the use of moist hand-formed loafs of clay-rich mud and straw which are mixed on-site, stacked up in courses to form walls, and often sculpted into the shapes of built-in fixtures such as shelves, benches, and bedding platforms. Wall finishes are usually natural plaster or adobe -plain or waxed- but materials like isochanvre can also be used. Flooring may be poured adobe, stone, tile, concrete, or most anything else that can be applied on top of those. Rustic wood plank flooring is the norm for upper floors.
Cob buildings are typically built on a stacked stone or concrete foundation fairly high above grade level -compared to typical foundations- and are topped-off with sloped roofing with a wide overhang to protect the cob walls from rain. Roofing type varies but contemporary designs favor wood shingle and slate roofing -often with decorative pattern treatments- on natural peeled log beams while the British tradition favors slate or thatch roofing on post & beam trusses. Very large structures commonly supplement the cob primary structure with a post & beam structure to support large spans and numerous storeys.
In the British tradition cob structures follow rectilinear building forms and can sometimes be several storeys high. In the contemporary style, single or two storey cottages with curving forms, small rooms, and elaborate sculptured details predominate. Curving forms are stronger than rectilinear forms, allowing novice owner-builders to produce sufficiently strong structures despite their inexperience with the material and building in general. In drier climates cob can be used in manners identical to adobe and in some cases has been used to make domed roof structures, though usually with another material -such as elastomerics or special hydrostatic plasters- added as a protection and some kind of reinforcement lattice to help support the structure under construction.
Though based on earthen material, cob buildings have proven practical in a very diverse range of climates including the notoriously damp climates of the UK and Oregon. This is due to the use of those wide roof overhangs and high foundations, as opposed to the flat roofs common to the pueblo style of adobe construction which cannot protect the structure sufficiently in persistent damp weather.
As a building method for non-toxic housing, cob is very effective as long as the earth sourced for construction is free from contamination -which is not always a given in the US. Offering the typical benefits of all earthen construction, cob buildings enjoy low energy costs due to high thermal mass and the materials normally used are naturally non-toxic, hydroscopic (meaning it absorbs and releases moisture in response to ambient humidity averaging out extremes), and breathable. (allowing a small slow amount of air flow through walls and their finishes) From a material standpoint cob is 'dirt cheap' but in practice such homes are only inexpensive when owner-builders have the physical ability and free time to perform most construction themselves. Labor overhead for cob construction is very high due to its heavy reliance on hand work and while even the most inexperienced can often produce serviceable structures, some skill and talent is necessary to realize attractive results. If one cannot build oneself, a cob home -like most earthen structures- can cost much more than a conventional stick-built homes. Cob also requires a routine schedule of maintenance in restoring the exterior surface finish due to weathering and this maintenance increases with the degree of dampness and temperature variation in the regional climate.
Cob homes very often rely on low-tech utilities, in particular wood stoves and fireplaces for heating. These are inadvisable for non-toxic housing which should normally avoid all forms of combustion energy appliances.
For more info see; The Cob Cottage Company